Safety-belt arrangement

ABSTRACT

A safety-belt arrangement for use in a motor vehicle has a safety-belt ( 5 ), one end of which is connected to a retractor ( 8 ), a sensor ( 11 ) generates a signal representative of the amount of safety-belt paid-out from the retractor. A force limiter ( 10 ) is associated with the retractor to permit safety-belt to be paid-out with a variable force limiting effect. A control arrangement ( 12 ) changes the level of force applied by the force limiter ( 10 ) in dependence upon the length of safety-belt paid-out from the retractor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

THE PRESENT INVENTION relates to a safety-belt arrangement, and moreparticularly relates to a safety-belt arrangement intended for use in amotor vehicle.

2. Description of Related Art

A simple safety-belt arrangement, as in widespread use, comprises alap-and-diagonal safety-belt arrangement, (or “three-point” safety-belt)with one end of the safety-belt being anchored to part of the vehicle,an intermediate part of the safety-belt being provided with a tonguereleasably engageable with a fixed buckle, and the other end of thesafety-belt, which may pass through a pillar-loop, being wound on to thespindle of a retractor. The retractor incorporates a mechanism whichlocks the spindle of the retractor to prevent further safety-belt beingpaid out from the retractor when an accident or a potential accidentsituation is sensed. If the safety-belt is locked in this way, however,the safety-belt can, in certain circumstances, apply a very greatretarding force to the occupant of the vehicle, decelerating theoccupant very swiftly, and also possibly injuring the occupant.

Thus it has been proposed to provide a force limiter in a safety-beltsystem, the force limiter being adapted to enable a certain length ofsafety-belt to be paid-out, with a force limiting or energy absorbingeffect, when very high forces are applied to the safety-belt.

It has been proposed to provide an arrangement in which the forcelimiting effect can be varied, at the start of or during an accidentsituation, in dependence upon the load required to be absorbed by thesafety-belt arrangement. An arrangement of this type is disclosed in WO9749583-A in which the reel of a retractor is provided with a torsionelement arrangement which provides the force limiting effect. Thetorsion element arrangement includes an axially extending torsion barand a co-axial torsion sleeve. The torsion bar and the torsion sleeveeach terminate with a portion having a respective toothed peripherywhich can be releasably engaged by a respective ratchet element. In thisway the level of force of the force limiting effect can be selected, byselecting either the bar, or the sleeve, or both, to provide the forcelimiting effect.

It is desirable, in a typical accident situation, for the force limiterto provide a high force limiting effect during the initial stages of theaccident, as the occupant of the seat begins to move forwardly relativeto the chassis of the vehicle, and to provide a lower force limitingeffect at a subsequent stage in the accident when the occupant has movedforwardly and begins to impact with an inflating air-bag. It has,therefore, been proposed to have an arrangement in which the force level(or energy absorbing level) of the force limiter is adjusted after apredetermined period of time has elapsed following the sensing of anaccident or a potential accident situation, and thus after thepretensioner has been triggered. However, an arrangement of this typesuffers from various disadvantages.

Many accidents start with minor or short interaction with a firstobject, such as an impact with a car having a relatively low speed, oran impact with the edge of the road, or a crash barrier, with this minoror short interaction being followed, some moments later, by a severeimpact with a second object, such as an on-coming high speed car, a treeor a building. In such a situation the first interaction would be sensedby the sensor provided in the motor vehicle to detect an accident or apotential accident situation. The sensor may control various safetydevices within the motor vehicle, such as a pre-tensioner and anair-bag, but would also start the operation of the timer associated withthe force limiter.

In an accident of this type, the force limiter would exhibit a highlevel of force for a period of time following the minor or shortinteraction, but the occupant of the vehicle may not move forwardly atall, or may move forwardly only by a very short distance during thisperiod of time. Before the severe impact with the second object, thetimer may have timed-out, and thus the force limiter would only have thelower level of force. Then, when the severe impact occurs, which willcause the occupant to move forwardly, relative to the chassis, the levelof force provided by the force limiter might well be too low to absorball of the energy of the occupant before the occupant hits the steeringwheel or the dashboard.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved safety-beltarrangement.

According to this invention there is provided a safety-belt arrangementfor use in a motor vehicle, the safety-belt arrangement comprising asafety-belt, one end of which is connected to a retractor, a sensoradapted to generate a signal representative of the amount of thesafety-belt paid-out from the retractor, a force limiter associated withthe retractor adapted to permit the safety-belt to be paid-out with avariable force limiting effect, means to change the level of forceapplied by the force limiter, and a control unit to receive a signalfrom the sensor and to generate, at least partially in response to saidsignal, an output signal to control the means which change the level offorce.

Preferably the retractor incorporates a pretensioner.

Conveniently the control unit generates said output signal in responseto the amount of safety-belt paid-out in excess of the amount ofsafety-belt paid-out prior to activation of the pretensioner.

Alternatively the control unit generates said output signal in responseto the amount of safety-belt paid-out in excess of the amount ofsafety-belt paid-out immediately after the pretensioner has beenactivated.

In a preferred embodiment means are provided to lock the safety-belt inresponse to a sensed parameter and the control unit generates saidoutput signal at least partly in response to a predetermined amount ofsafety-belt being paid-out in excess of a reference length, means beingprovided to determine the reference length at or shortly after theinstant of locking of the safety-belt.

In one embodiment a crash detector is provided and the means todetermine the reference length are adapted to be actuated apredetermined period of time after the crash detector detects the crash.

The predetermined period may be 20 to 30 ms, and preferably is 20 ms.

Preferably the control unit is adapted to generate the control signal apredetermined period of time after the predetermined amount ofsafety-belt has been paid out.

In one embodiment means are provided to select the predetermined periodof time in response to a signal indicative of the severity of the crash.

Conveniently a further sensor is provided to generate a signalrepresentative of a parameter of the occupant, which signal is suppliedto the control unit, the output signal of the control unit also beingrelated to the signal from the second sensor.

In one embodiment the further sensor determines the weight of theoccupant of the seat.

In another embodiment the further sensor is a capacitative sensoradapted to determine the water content of the occupant of the seat

In a further embodiment the further sensor is an optical sensor adaptedto determine a size parameter of the occupant of the seat.

Preferably the control unit is adapted to categorise an occupant, inresponse to the signal representative of a parameter, into one of aplurality of categories, the length of safety-belt necessary to bewithdrawn to cause the change in the force limiting effect beingdependent upon the category of the occupant of the seat.

Conveniently the said variable force limiting effect comprises aplurality of discrete force levels.

Preferably the force levels comprise two discrete force levels.

Advantageously a crash sensor is provided adapted to generate a signalrepresentative of the crash severity, and the force limiting effect isadjusted to be a function of crash severity.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood, and so thatfurther features thereof may be appreciated, the invention will now bedescribed, by way of example, with reference to the accompanyingdrawings in which:

FIG. 1 is a diagrammatic figure illustrating one embodiment of theinvention,

FIG. 2 is a graphical figure showing a plot of force applied to asafety-belt against the length of belt withdrawn for a typical personprovided for purposes of explanation,

FIG. 3 is a graphical figure showing a plot of force applied to asafety-belt and length of belt extracted against time for a situationequivalent to that shown in FIG. 1,

FIG. 4 is a further graphical figure corresponding to FIG. 2 for a heavyperson,

FIG. 5 is further graphical figure corresponding to FIGS. 2, 3 and 4 fora light person, and

FIG. 6 is another graphical Figure relating to a modified embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring initially to FIG. 1 of the accompanying drawings, a vehicleseat 1 having a seat squab 2 and a back-rest 3 is illustrated. The seatis occupied by an occupant 4. The occupant 4 is illustrated wearing asafety-belt that forms a safety-belt arrangement in accordance with theinvention. The safety-belt comprises a webbing strap, one end of whichis anchored to part of the vehicle seat. Not shown in FIG. 1 is a tonguewhich is mounted on the safety-belt and which is engaged releasably in abuckle provided on the other side of the seat. Part of the safety-belt 5passes through a pillar-loop 7, and the other end of the safety-belt isconnected to a floor mounted retractor 8. The retractor 8 is adapted torespond to an accident situation to lock a spool within the retractor toprevent further safety-belt being paid-out from the retractor, bylocking in response to a sensed parameter such as a predetermineddeceleration of the vehicle and/or in response to the speed ofwithdrawal of the safety-belt from the retractor. The retractor isprovided with a pretensioner 9 which is adapted, in response to anaccident situation being sensed, to rotate the spool of the retractor towind in part of the safety-belt. Safety-belt arrangements having thefeatures described above are well known and in common use. However, inthis invention alternative mechanisms which lock the safety-belt inresponse to a sensed parameter may be used.

In the described embodiment the retractor 8 incorporates a variableforce limiter 10. The force limiter 10 may be a continuously variableforce limiter, such as an adjustable brake mechanism which acts on adisc or drum which co-rotates with the spool of the retractor mechanism,or may be a variable force limiter which provides a plurality ofdiscrete force levels, such as a torsion element force limiter of thetype disclosed in WO 9749583-A, as described above.

The retractor 8 is provided with a sensor 11 adapted to sense the amountof safety-belt that has been withdrawn from the retractor. Various typesof sensors which perform this function have been proposed before, andany of these sensors may be used. EP 0 723 895 A discloses anarrangement in which a gear train is provided, driven by the spooi of aretractor, which causes the rotation of an element which has a partspiral outer edge. A sensor has a spring-biased probe which engages thepart spiral outer edge, so that as safety-belt is withdrawn from theretractor, the element with the part spiral outer edge rotates and thedegree of movement of the probe is indicative of the length ofsafety-belt that has been withdrawn. In another prior proposedarrangement, an element which co-rotates with the spool is provided witha plurality of evenly spaced markings which are sensed by an opticalsensor. The optical sensor generates a discrete pulse for each markingthat passes the sensor. Appropriate circuitry connected to the sensorcan determine the length of safety-belt that has been paid-out bydetermining the number of rotations, or part rotations, of the spool. AHall effect sensor can also be used to determine the rotations of thespooi when the spool is provided with a number of appropriately locatedmagnets.

The output from the sensor 11 is provided to a central control unit 12.The central control unit 12, in the described embodiment, also receivessignals from other sources. The control unit 12 receives signals from anaccident sensor 13 in the form of an accelerometer. The accelerometerprovides an output signal whenever deceleration in excess of apredetermined threshold is sensed. The accelerometer may also provide asignal which indicates the severity of the crash. Such an accidentsensor 13 is conventional. The control unit 12 also receives signalsfrom a further sensor arrangement, which signals are representative of aparameter of the occupant of the seat. In the described embodiment ofthe invention, a weight sensor 14 is provided which is incorporatedwithin the squab 2 of the seat adapted to generate a signal indicativeof the weight of the occupant 4. Such a sensor may includepiezo-electric sensor devices. In alternative embodiments of theinvention, the seat 1 may include a plurality of capacitative plateswhich are connected to appropriate circuitry, and which are adapted todetermine the change in capacitance between the capacitative plates whenthe occupant of the seat positions himself on the seat. The change incapacitance is a function of the total water content of the occupant ofthe seat, which is closely related to the total weight of the occupantof the seat. Thus, by using an arrangement of this type, it is againpossible to generate a signal which is at least approximately related tothe weight of the occupant of the seat.

In yet a further embodiment of the invention, instead of determining theweight of the occupant of the seat, an optical system may be used whichdetermines, optically, the size of the occupant or the size of at leastpart of the occupant. This, again, will generate a signal which is atleast proportional to the weight of the occupant of the seat.

In the presently described embodiment, the control unit 12 is adapted tocontrol not only the pre-tensioner and the force limiter, but also anair-bag 15, which is located in front of the occupant.

Whilst, in the described embodiment, the single control unit 12 controlsa number of different items, in alternative embodiments of theinvention, the control function may be divided up between a number ofdifferent independent control units.

As will become clear from the following description, in a safety-beltarrangement in accordance with the invention, a variable force limitingeffect is provided, with the variation in the force limiting effectbeing controlled in response to the length of safety-belt that has beenpaid-out, rather than the time from the commencement of the accident orpotential accident situation. Also, in preferred embodiments of theinvention, the length of safety-belt that has to be paid-out to effect achange in the force limiting effect is determined in dependence uponsensed parameters of the occupant, and also possibly in response to asignal from the accelerometer indicating the degree of severity of theaccident.

Referring now to FIGS. 2 and 3, which is a graphical figure, theoperation of the safety-belt arrangement of FIG. 1 in a typical accidentfor a person of typical or normal weight will be described. The graph ofFIG. 2 is a plot of the force of the force limiting effect as plottedagainst the length of safety-belt withdrawn from the retractor. Thegraph of FIG. 3 is a plot of force and length of safety-belt withdrawnagainst time, with increasing length shown towards the foot of the page.

It can be seen that there are two predetermined retarding forces shownon the graph F₁ and F₂, the force F₂ being greater than the force F₁.These forces may, by way of example, be considered to be the forcesprovided by the torsion bar or the torsion tube, respectively, of anarrangement as shown in WO 9749583-A.

The accident situation commences at point 20 on the graph. At this stagea predetermined length of safety-belt has been withdrawn from theretractor. This is the length of safety-belt withdrawn as the occupantplaces the safety-belt in the position illustrated in FIG. 1. In thisexample this length of safety-belt is the length of safety-beltordinarily deployed when the occupant who is of normal or typical size,is sitting comfortably in an upright position, and is identified as L₀.When an accident, or potential accident situation is sensed by theaccelerometer 13, when the change in velocity of the vehicle exceeds apredetermined threshold, at time to, a signal from the control unit 12is passed to the pretensioner 9 which is activated to wind insafety-belt on to the spool of the retractor 8. The spool of theretractor thus rotates and if desired the sensor 11 could sense thatsafety-belt is being wound in, against a gently increasing resistiveforce which gradually rises to a level F_(p) until the point 21 isreached, when the safety-belt is fully tight around the occupant. Thelength of belt withdrawn from the retractor is now at a minimum L_(min).The time is now t_(min). The period of time between and t₀ and t_(min)is a as shown in FIG. 3. This time period represents the time taken forthe pretensioner to pretension the webbing. For the purposes ofexplanation, it will be assumed that the accident sensed by theaccelerometer 13 is an initial impact with the side of the road or acrash barrier. The deceleration of the chassis of the vehicle is onlyvery small, and thus the occupant does not move forward, relative to thechassis, by any noticeable distance as a consequence of the impact withthe side of the road or the crash barrier. However, since the accidenthas been sensed by the accelerometer, the safety-belt has been tensionedby the pre-tensioner, and the occupant has been pulled backwardly,relative to the chassis, by the pre-tensioner.

When the pre-tensioner has finished pulling the webbing, the tensionforce on the webbing will reduce and due to the elasticity of thewebbing, and also due to the elasticity of the clothes and body of theoccupant, webbing will be paid-out. Thus, clothes of the occupant whichwere initially compressed during the pre-tensioning phase may nowre-expand. Thus the length of safety-belt paid-out increases while thetension force on the safety-belt reduces.

As the webbing is paid-out, the locking mechanism of the retractor willoperate, and the retractor will become locked at time t_(lock) as shownat point 22 in FIG. 2. The period of time running from t_(min) tot_(lock) has a duration b as shown in FIG. 3.

At time instant t_(lock), the retractor becomes locked by the lockingmechanism that forms part of the retractor. Typically t_(lock) may be 15to 19 ms after t₀. The precise time period between t₀ and t_(lock) maydepend upon factors such as variations in the weight of the seatoccupant, or the severity of the accident, but mainly upon the specificdesign and functioning of the pre-tensioner.

t_(lock) may occur shortly before, simultaneously with, or shortly afterthe main impact of the accident presently being considered. During thismain impact the chassis of the vehicle is subjected to severedeceleration, and possibly also acceleration in the rearward direction,especially if the impact is an impact with a high speed vehicletravelling in the opposite direction.

During this phase of the accident, the webbing of the safety-belt actsto decelerate the occupant, and possibly acts to accelerate the occupantrearwardly, depending upon the movement of the chassis. During thisphase the force in the webbing is increased until the force reaches thehigher level F₂. When the force reaches the level F₂, the force limiterstarts to pay-out webbing. The rate at which the webbing is paid-out isrelated to the integral of the relative acceleration (a_(rel)) betweenthe chassis and the occupant. The relative acceleration (a_(rel)) is thedifference between the acceleration of the chassis (a_(c)) and theacceleration of the occupant (a₀). Thus:a _(rel) =a _(c) −a ₀.The acceleration of the chassis is greater than or equal to theacceleration of the occupant. The acceleration of the occupant isrelated to the force level F₂ divided by the mass of the occupant. Ifthe mass of the occupant is large, a₀ may be small, but a_(rel) may belarge. Thus typically webbing is paid-out at a higher rate for a heavieroccupant.

The time period between t_(lock) and the point where the resistive forceF₂ is reached, point 23 as shown on the graph, has a duration c, andthis time period is normally between 10 and 25 ms. This variation intime is typically due to the weight of the occupant, and the precisenature of the crash pulse, that is to say the period of time between thefirst impact and the main impact in the accident situation beingconsidered, and also the degree of deceleration or rearward accelerationapplied to the chassis to the vehicle in the particular crash situation.

It is thus to be understood that the period of time between t_(lock) andthe point 23, then the force F₂ is provided by the force limiter may,depending upon the precise circumstances, have a very differentduration, with a short period of 10 ms being experienced in somesituations, and a period of 100 ms being experienced in othersituations.

During this phase, between t_(lock) and point 23, however, the occupantof the seat will only move forwardly very little, and mainly due to theelasticity of the webbing.

During this phase, that is to say during the phase of movement withelastic stretching of the webbing following locking of the spool andbefore the occupant of the seat has stretched the webbing to such anextent that the full resistive force F₂ of the force-limiter isexperienced, the sensor which senses the length of safety-belt that hasbeen paid-out, takes a reference measurement at time t_(ref), andeffectively measures or sets a reference length of safety-belt that hasbeen paid-out L_(ref). This reference length is used as a start pointwhen measuring a length of belt to be paid out against a specificresistive force F₂ as will now be described.

The point at which the reference measurement is made is shown as point24 on the graph of FIG. 2. In making the measurement, t_(ref) to be aselected period of time, preferably within the time period of 20–30 ms,most preferably within a time period of 20 ms, after t₀ to ensure thatthe time t_(ref) will actually be in the time period c followingt_(lock), or, alternatively, the measurement at time t_(ref) may be madein response to locking of the retractor reel.

If the occupant then moves forwardly, after the point 23 on the graph isreached, the resistive force F₂ is experienced as the safety-belt ispaid-out. This is shown by the horizontal line 25 of the graph. The timetaken for the occupant to move forwardly, against the resistive force,is not of any relevance since the resistive force F₂ will be enableduntil a predetermined length of belt has been paid-out, with the lengthbeing measured from the reference length L_(ref). After a predeterminedlength of safety-belt has been paid-out, as measured from the referencelength, L_(ref) at point 26 on the graph, and identified as L_(ch), theforce limiter 10 no longer provides the high resistive force F₂, butinstead provides the lower resistive force F₁, as shown at point 27 onthe graph. This can be visualised as changing from the torsion bar tothe torsion tube of the arrangement of WO 9749583-A. Continued forwardmovement of the occupant is then effected against the much lowerresistive force as shown at line 28. In a typical case, during thisphase of movement of the occupant of the seat, energy is also beingabsorbed, as shown in phantom at line 29, by an inflating air-bag.

It is to be appreciated that the change of the force level of the forcelimiter occurs when a certain amount of safety-belt has been paid-out,from the measured length L_(ref) and thus occurs when a certain amountof energy has been absorbed. In the arrangement described, the length ofsafety-belt that is paid-out is measured from a reference point which iseffectively the length of safety-belt paid-out at the moment that theretractor is locked. As the length of belt withdrawn during the periodof duration c between t_(lock) and when force F₂ is achieved is veryshort, any reference measurement taken in this time period will besufficiently accurate. The instant of this “locking” is slightlyuncertain in time as regards the instant of initial sensing of anaccident situation, and the instant at which the resistive force F₂ isachieved is more uncertain, and thus it is appropriate to use the momentat which the retractor becomes blocked, or an instant which is, in time,very shortly after to that moment, as a reference point at which todetermine L_(ref). Thus, provided that the distance L_(ref) isdetermined during the time period between t_(lock) and point 23 when theresistive force F₂ is achieved, the measured point L_(ch) will be almostprecisely at the desired position, and the energy absorbed before thechange of force will therefore also be almost exactly the desiredamount, regardless of the duration of the time after commencement of theaccident before force F₂ is achieved. The energy absorbed is related tothe length of belt withdrawn multiplied by the force.

The safety-belt is withdrawn until a maximum extension L_(max) isachieved, at which point all of the kinetic energy of the occupant isabsorbed. L_(max) is selected so that the kinetic energy of the occupantis absorbed before the occupant actually reaches the steering wheel orwindscreen.

The transition between the high energy absorbing level F₂ and the lowenergy absorbing level F₁, as shown at 26 and 27 on the graph, iseffected when a predetermined length L_(ch) of safety-belt has been paidout. That predetermined length of safety-belt is, as described,preferably measured from L_(ref), but could alternatively be measuredfrom L₀, the amount of safety-belt withdrawn from the retractor justbefore the commencement of the accident situation, or from L_(min), theamount of safety-belt withdrawn from the retractor just after thepretensioner has pre-tensioned the safety-belt.

It is to be appreciated that during the final part of the withdrawal ofthe safety-belt, the occupant is restrained, not only by the safety-beltbut also by the air-bag, and thus the speed of withdrawal of thesafety-belt remains substantially constant, and the total retardingforce applied to the occupant remains substantially constant. FIG. 3shows, in phantom, at 30, the situation that would prevail if theair-bag did not inflate.

It is preferred that when the occupant of the seat is a heavy or largeoccupant, the length of safety-belt is paid-out against the highresistive force should be somewhat greater than for an occupant ofnormal weight. FIG. 4 is therefore a graph corresponding to FIG. 2, andin which the same reference numerals have been utilised, but whichrelates to a heavy seat occupant.

As can be seen from FIG. 4, the initial point on the graph shows alength of belt L_(ob) withdrawn from the spool, which is less than thelength L₀ as withdrawn in FIG. 2. The reason for this is that with aheavy or large occupant, the vehicle seat will be moved further awayfrom the steering wheel, meaning that less safety-belt has to bewithdrawn initially. However, it can equally be seen that the distancebetween L_(ob) and L_(max) and also the distance between L_(ref) andL_(chb) is much greater than the equivalent distances in FIG. 2. Thedistance between L_(ref) and L_(chb) is greater, to enable more energyto be absorbed. The distances are, however, still such that at L_(max)the occupant has not actually reached the steering wheel or windscreen.

Referring now to FIG. 5 it is to be understood that for a light-weightoccupant it may be desirable to omit the very strong energy absorbingforce, and to use only the relatively low force. Thus, referring to FIG.5, it is to be noted that the initial extension L_(os) of safety-belt isgreater than the extension L₀ of FIG. 2. The reason for this is that fora light-weight occupant the vehicle seat will be located closer towardsthe steering wheel than for an occupant of normal weight, meaning thatmore safety-belt will have to be withdrawn, initially, from theretractor for the safety-belt to be placed in position.

Following commencement of the accident at point 20, safety-belt iswithdrawn from the initial extension L_(os) to an extension L_(min), asshown at point 21. During the main crash pulse, that is to say duringthe time period c, the safety-belt is pulled out from the retractor andthe resistive force rises until it reaches the force level F₁ at 23.Subsequently the occupant can move forwardly withdrawing thesafety-belt, as shown at 25, against the resistive force F₁ until themaximum extension of safety-belt L_(max) is achieved. It is to beappreciated that much less energy is absorbed during the describedprocedure, since only the relatively low resistive force F₁ is utilised.However, with a light-weight occupant, this is a sufficient degree ofenergy absorption, since it is only necessary to use the higher forcelevel F₂ for a person of normal, or above normal weight.

FIG. 6 is a graphical Figure which relates to an embodiment in which thecontrol unit generates the control signal not only in response to apredetermined amount of safety-belt having been paid out after theretractor reel is locked, but in response to the paying out of aspecific length of safety belt, and also a subsequent predeterminedperiod of time. That period of time may be adjusted in response tosensed crash conditions, such that the period of time is related to theseverity of the crash, and thus the described arrangement can provide anoptimum effect. Thus, in this embodiment the control unit 12 willdetermine when a predetermined length (L₁) of safety belt has been paidout following t_(ref) which is very shortly after locking of theretractor reel, and will then start an internal timer which times apredetermined time period (T₁) so that the control signal that causesthe change of the resistive force from the higher level F₂ to the lowerlevel F₁.

FIG. 6 shows, for a given severity of accident, the length ofsafety-belt withdrawn from the retractor reel of this embodiment plottedagainst time, with increasing length shown towards the front of thepage, and thus the Figure has some similarity to that of FIG. 3. Thelength of safety-belt withdrawn from the retractor falls following thecommencement of the accident at t₀ at point 20 on the graph. At point 21the safety-belt is fully tight around the occupant of the vehicle, andtime period a concludes and time period b begins. During time period bthe occupant may move forwardly slightly in the seat and time period bends at point 22 on the graph as the retractor reel locks at t_(lock).Shortly after the reel locks at t_(ref) as shown at point 24, the lengthof safety-belt L_(ref) withdrawn from the retractor is determined orset. During the period of time following t_(lock) the occupant continuesto be pushed forwardly stretching the webbing against a restrainingforce that rises to the level F₂ during the time period c. This timeperiod is relatively short for a heavy person, as exemplified by line30, the time period being about 10 ms, as the high restraining force isreached quickly following the locking of the retractor. The time periodc for the heavy person identified by line 30 terminates as shown by line40, and then the safety-belt is pulled out against the retarding forceF₂ and is identified as C_(min). The line 50 shows the desired lengthL_(ch) for a heavy person.

Looking now at the example of a light person, as indicated by the line34, it can be seen that the time period c is relatively long, and doesnot end until the point indicated by the line 44. This time period isidentified as C_(max) The line 54 shows the desired length L_(ch) for alight person.

It can be seen that if the time is measured between the point at which apredetermined length of safety-belt L₁ after L_(ref) has been withdrawn,until the line representing the desired length L_(ch) is reached, ineach case the time is the same, namely T₁. It can also be seen that thesame effect is observed for occupants of decreasing weight as indicatedsuccessively by the lines 31, 32 and 33. The desired length L_(ch) foreach example are shown to be interconnected by the line 60.

Thus, at least to a good approximation, the length L_(ch) is reachedafter a first predetermined length L₁ of safety belt has been withdrawn,and a further length of belt has been withdrawn during a time period T₁.Consequently by determining when the length L₁ of safety-belt has beenwithdrawn, and then measuring the time T₁ to generate the control signalto effect the change of the resistive force it is not necessary toeffect any measurement of any parameter of the occupant related to thesize of the occupant, thus minimising the number of sensors that need tobe provided.

The pay-out rate of the safety-belt will be higher for a more severecrash, and thus the described system is automatically adaptive to theseverity of the crash. If the system is to be such that it responds andvaries its performance not only in response to the severity of thecrash, but also in dependence on the weight of the occupant, then theprecise values of L₁ and T₁ may be varied in dependence on the sensedseverity of the crash.

Whilst reference has been made, in connection with the graphs of FIGS.3, 4, and 5 to different operating characteristics being provideddepending upon the weight of the occupant of the seat, it is to be fullyunderstood that these different characteristics may be provided inresponse to any sensed parameter which is related to the weight of theoccupant of the seat such as a sensed parameter of the water content ofthe occupant of the seat, and a sensed parameter relating to the size ofthe occupant of the seat.

It is to be appreciated that in certain embodiments, the actual level offorce exhibited by the force limiter may vary continuously, although inpreferred embodiments of the invention, the force limit level can onlychange between a limited number of discrete levels. Whilst embodimentshave been disclosed in which the force limit level can be changedbetween two discrete levels, it is to be understood that the force limitlevel can be changed between a plurality of discrete levels. It is to beappreciated that the force limit level actually exhibited by the forcelimiter may be determined as a function of the crash severity. Thus, ina very severe accident, an initial force limit level may be adoptedwhich is a very high level, which can be visualised as usingsimultaneously both the torsion bar and the torsion tube of WO9749583-A.

For simplicity of manufacture, the relationship between the weight orother parameter of the occupant and the functioning of the force limitermay be such that the force limiter has a restricted number of discretemodes of operation, such as, for example, three modes of operation asdescribed. Thus, an occupant of a seat will be assessed, by theappropriate sensor, to determine which category of occupant is presentin the seat, and thus which mode of operation is to be utilised.

Thus, in the example given, an occupant of a seat may be assessed asbeing “light”, “normal weight” or “heavy”, and the described arrangementwill then function in the appropriate manner.

In the present Specification “comprises” means “includes or consists of”and “comprising” means “including or consisting of”.

1. A safety-belt arrangement for use in a motor vehicle, the safety-beltarrangement comprising a safety-belt, one end of which is connected to aretractor, a sensor adapted to generate a signal representative of theamount of the safety-belt paid-out from the retractor, a force limiterassociated with the retractor adapted to permit the safety-belt to bepaid-out with a variable force limiting effect, means to change thelevel of force applied by the force limiter, and a control unit toreceive a signal from the sensor and to generate, at least partially inresponse to said signal, an output signal to control the means whichchange the level of force, and wherein means are provided to lock thesafety-belt in response to a sensed parameter and wherein the controlunit generates said output signal at least partly in response to apredetermined amount of safety-belt being paid-out in excess of areference length, wherein the sensor determines the reference length ator shortly after the instant of locking of the safety-belt.
 2. Anarrangement according to claim 1 wherein the retractor incorporates apretensioner.
 3. An arrangement according to claim 2 wherein the controlunit generates said output signal in response to the amount ofsafety-belt paid-out in excess of the amount of webbing paid-out priorto activation of the pretensioner.
 4. An arrangement according to claim2 wherein the control unit generates said output signal in response tothe amount of safety-belt paid-out in excess of the amount ofsafety-belt paid-out immediately after the pretensioner has beenactivated.
 5. An arrangement according to claim 1 wherein a crashdetector is provided and the means to determine the reference length areadapted to be actuated a predetermined period of time after the crashdetector detects the crash.
 6. An arrangement according to claim 5wherein the predetermined period is 20 to 30 ms.
 7. An arrangementaccording to claim 6 wherein the period is 20 ms.
 8. An arrangementaccording to claim 1 wherein the control unit is adapted to generate thecontrol signal a predetermined period of time after the predeterminedamount of safety-belt has been paid out.
 9. An arrangement according toclaim 8 wherein means are provided to select the predetermined period oftime and the predetermined amount of safety-belt in response to a signalindicative of the severity of the crash.
 10. An arrangement according toclaim 1 wherein a further sensor is provided to generate a signalrepresentative of a parameter of the occupant, which signal is suppliedto the control unit, the output signal of the control unit also beingrelated to the signal from the second sensor.
 11. An arrangementaccording to claim 10 wherein the further sensor determines the weightof the occupant of the seat.
 12. An arrangement according to claim 10wherein the further sensor is a capacitative sensor adapted to determinethe water content of the occupant of the seat.
 13. An arrangementaccording to claim 10 wherein the further sensor is an optical sensoradapted to determine a size parameter of the occupant of the seat. 14.An arrangement according to claim 10 wherein the control unit is adaptedto categorize an occupant, in response to the signal representative of aparameter, into one of a plurality of categories, the length ofsafety-belt necessary to be withdrawn to cause the change in the forcelimiting effect being dependent upon the category of the occupant of theseat.
 15. An arrangement according to claim 1 wherein the said variableforce limiting effect comprises a plurality of discrete force levels.16. An arrangement according to claim 15 wherein the force levelscomprise two discrete force levels.
 17. An arrangement according toclaim 1 wherein a crash sensor is provided adapted to generate a signalrepresentative of the crash severity, and the force limiting effect isadjusted to be a function of crash severity.
 18. A safety-beltarrangement for use in a motor vehicle, the safety belt arrangementcomprising: a safety-belt having an end; a retractor capable of payingout the safety belt, the end of the safety-belt connected to theretractor; a sensor capable of measuring an amount of safety-beltpaid-out from the retractor, the sensor generating a correspondingsignal; a force limiter associated with the retractor, the force limiterpermitting the safety-belt to be paid-out with a variable force limitingeffect, the force limiter having a force changing mechanism capable ofchanging the level of force applied by the force limiter; a control unitable to receive the signal from the sensor, the control unit generatingan output signal to control the force changing mechanism in response tothe sensor signal; and a locking mechanism for locking the safety-beltin response to a sensed parameter; wherein the sensor determines areference length at or shortly after the instant of locking thesafety-belt, and the control unit generates the output signal inresponse to a predetermined amount of safety-belt paid-out in excess ofthe reference length.
 19. An arrangement according to claim 18, furthercomprising a crash detector for detecting a crash, wherein the sensordetermines the reference length at a predetermined period of time afterthe crash is detected.
 20. An arrangement according to claim 18, whereinthe control unit is able to generate a control signal at a predeterminedperiod of time after the predetermined amount of safety-belt has beenpaid-out.
 21. An arrangement according to claim 20, wherein the controlunit selects the predetermined period of time and the predeterminedamount of safety-belt to be paid-out in response to a signal indicativeof the severity of a crash.
 22. An arrangement according to claim 18,wherein the force changing mechanism is a torsion element comprising atorsion bar and a torsion tube.
 23. An arrangement according to claim18, wherein the force changing mechanism is an adjustable brakemechanism.